This website links to more than 500 libraries in the world which contain medieval manuscripts that can be browsed for free and includes a useful map function to pinpoint different libraries and their holdings.
Early English Books Online (EEBO) contains images (and where available also digitised texts) of dozens of thousands of books printed between 1473 and 1700; it covers almost anything printed in England, Ireland, Scotland, Wales and British North America during that time, as well as English-language works printed elsewhere.
Digita Vaticana Onlus is a non-profit organization founded in 2013 to promote the conversion of the Vatican Library manuscripts into digital format. The Association is raising the necessary funds to guarantee the digitization of over 80,000 manuscripts held in the Vatican Library.
Includes medieval manuscripts, Mercator globes, the Islamic Heritage Project, early modern and modern crime broadsides, and a collection focusing on the history of reading ("Reading: Harvard Views of Readers, Readership and Reading History").
Many different collections including the Rylands Collection, Early Printing Collection, Dante Collection (early printed editions), Hebraica Collection (manuscripts, amulets, archives, and printed books), and Genizah Collection (over 15,000 fragments).
The Munich DigitiZation Center digital collections, containing items from the Bayerische Staatsbibliothek (Munich) and other institutions. Contains over 2,000,000 items, including manuscripts, early printed books, and maps. Can be search by format, period, geographical region, publishing houses, and similar images.
Includes several digital collections relevant for book history and art history research, including manuscripts and rare books, early modern craftsmen's guild books, and early library receipt books (borrowing registers).
Several digital collections including the Stavros Niarchos Foundation Digital Library (Greek literature and archaeology), the UCL School of Slavonic and East European Studies Library (early modern maps and travel books), and Medieval manuscripts fragments.
Contains digitised copies of many of the Wellcome Library's 1,000 Arabic manuscripts, most of them pertaining to the history of medicine.
See also http://wellcomelibrary.org/collections/browse/collections/digarabic/
Contains links to several digital collections of medieval and early modern books, including Arabic Manuscripts, Medieval Manuscripts, and Recipe Books.
To see full collections, go to "Browse Digitised Content".